Method for preparing a masterbatch of diene elastomer and silica

ABSTRACT

Preparation of a masterbatch of diene elastomer and silica including preparing at least one silica dispersion in water; bringing into contact and mixing an elastomer latex and the aqueous silica dispersion in the presence of a calcium salt in order to obtain a coagulum; and recovering the coagulum and drying the recovered coagulum in order to obtain the masterbatch. The molar content of calcium cations, defined as the number of moles of calcium cations of the calcium salt per BET unit area of the silica, is between 1.19*10 −6  and 2.81*10 −6  mol/m 2 .

This application is a 371 of PCT/EP2012/070031, filed 10 Oct. 2012,which claims benefit under 35 U.S.C. §119 of the filing date of Frenchpatent application 1159178, filed 11 Oct. 2011, the entire contents ofwhich is incorporated herein by reference for all purposes.

BACKGROUND

1. Field

The disclosure relates to the preparation of a masterbatch of dieneelastomer and silica comprising at least one silica, a calcium salt anda diene elastomer latex, in particular a natural rubber latex.

2. Description of Related Art

The term “masterbatch” is understood to mean an elastomer-basedcomposite into which a filler and optionally other additives have beenintroduced.

The present disclosure relates to in particular to the use of such amasterbatch in the manufacture of diene rubber compositions reinforcedwith an inorganic filler intended for the manufacture of tires orsemifinished products for tires, in particular treads of these tires.

It is known, in order to obtain optimum reinforcing properties conferredby a filler in a tire tread and thus a high wear resistance, that it isgenerally advisable for this filler to be present in the elastomermatrix in a final form which is both as finely divided as possible andas homogeneously distributed as possible. In point of fact, suchconditions can only be achieved insofar as this filler exhibits a verygood ability, on the one hand, to be incorporated in the matrix duringthe mixing with the elastomer and to deagglomerate and, on the otherhand, to disperse homogeneously in this matrix.

In a known way, carbon black exhibits such abilities, which is generallynot the case with inorganic fillers, in particular silicas. This isbecause, for reasons of reciprocal affinities, these inorganic fillerparticles have an unfortunate tendency, in the elastomer matrix, toagglomerate with one another. These interactions have the harmfulconsequence of limiting the dispersion of the filler and thus thereinforcing properties to a level substantially below that which itwould be theoretically possible to achieve if all the bonds (inorganicfiller/elastomer) capable of being created during the mixing operationwere actually obtained; these interactions tend, moreover, to increasethe consistency in the raw state of rubber compositions and thus torender their processability more difficult than in the presence ofcarbon black.

Ever since savings in fuel and the need to protect the environment havebecome a priority, it has proved necessary to produce tires having areduced rolling resistance without having a disadvantageous effect ontheir wear resistance.

This has been made possible in particular by virtue of the use, in thetreads of these tires, of novel rubber compositions reinforced withinorganic fillers, especially with specific silicas of the highlydispersible type, capable of competing, from the reinforcing viewpoint,with a conventional tire-grade carbon black, while giving thesecompositions a lower hysteresis, synonymous with a lower rollingresistance for the tires comprising them, and an improved grip on wet,snowy or icy ground.

Treads filled with such highly dispersible silicas (denoted “HD” or“HDS” for “highly dispersible” or “highly dispersible silica”), whichcan be used in tires having a low rolling resistance sometimes describedas “Green Tires” for the energy saving offered to the user (“Green Tireconcept”), have been extensively described. Reference will in particularbe made to Patent Applications EP 501 227, EP 692 492, EP 692 493, EP735 088, EP 767 206, EP 786 493, EP 881 252, WO99/02590, WO99/02601,WO99/02602, WO99/06480, WO00/05300 and WO00/05301.

These documents of the prior art teach the use of silicas of the HD typeexhibiting a BET specific surface of between 100 and 250 m²/g. Inpractice, an HD silica having a high specific surface acting asreference in the field of “Green Tires” is in particular the Zeosil 1165MP silica (BET specific surface equal to approximately 160 m²/g) sold byRhodia. The use of this Zeosil 1165 MP silica makes it possible toobtain good compromises as regards tire performance, in particular asatisfactory wear resistance and a satisfactory rolling resistance.

The advantage of using a silica having a high specific surface liesmainly in the possibility of increasing the number of bonds of thesilica with the elastomer and thus of increasing the level ofreinforcement of the latter. This is why it appears advantageous to use,in rubber compositions for tire treads, silicas having a high specificsurface, possibly greater than that conventionally used of the order of160 m²/g, in order in particular to improve the wear resistance of thesetreads. Nevertheless, the dispersibility of the filler and the increasein its specific surface are regarded as contradictory characteristics.This is because a high specific surface supposes an increase in theinteractions between filler items and thus a poor dispersion of thefiller in the elastomer matrix and also a difficult processability.

Another type of solution has been envisaged which consists, in order toimprove the dispersibility of the filler in the elastomer matrix, inmixing the elastomer and the filler in the “liquid” phase. To do this,recourse has been had to an elastomer in the latex form, which exists inthe form of elastomer particles dispersed in water, and to an aqueousdispersion of filler, that is to say a silica dispersed in water,commonly known as silica slurry. However, bringing the elastomer latexand the slurry into contact does not make it possible (not troublesomefor the position of the problem, which relates to the silica only) tohave coagulation within the liquid medium, which coagulation should makeit possible to obtain a solid which, after drying, results in theproduction of the desired masterbatch of elastomer and silica.

This is because the silica aggregates are typically hydrophilic innature and have an affinity with the water; thus, the silica aggregateshave more affinity with the water than with the elastomer particlesthemselves.

Different solutions have been provided to make it possible neverthelessto obtain this coagulation and a good dispersion of the filler in theelastomer matrix in the “liquid” phase by the combined use of an agentwhich makes it possible to increase the affinity between the elastomerand the silica, such as a coupling agent, and of an agent which makes itpossible to bring about solidification, thus coagulation, known ascoagulating agent.

Thus, for example, U.S. Pat. No. 5,763,388 provides for theincorporation of silica in the rubber latex by treating the silica witha coupling agent and mixing the silica, thus treated, in the presence ofconventional coagulating agents.

Patent EP 1 321 488 also intends to bring an aqueous dispersion withnegatively charged silica and a diene elastomer latex into contact withan emulsion comprising a polysulphide coupling agent, in the presence ofa coagulating agent, such as a polyamine.

Patent EP 1 323 775 also provides bringing an aqueous dispersioncomprising an inorganic filler, such as a silica, into contact with anelastomer latex in the presence of a coagulating agent which can becomposed, according the list envisaged in this document, of sodiumchloride, potassium chloride, salts, acids, and the like. Morepreferably, this method describes an additional stage which makespossible the in situ formation of the inorganic filler during the mixingbetween the aqueous dispersion comprising a precursor capable of formingthe inorganic filler and the latex, before adding a coagulating agent.

SUMMARY

The Applicant Companies have discovered, surprisingly, a simplifiedmethod for obtaining a silica/elastomer masterbatch prepared in the“liquid” phase using a specific salt in a low predetermined amount,without the need for the addition of other additives or coagulatingagents. This method makes it possible in particular to achieve a verygood level of yield (greater than 80% by weight) while observing thecontent of filler introduced beforehand and a good dispersion of thefiller in the elastomer matrix.

The method for preparing a masterbatch of diene elastomer and silica inaccordance with an embodiment of the invention comprises the followingsuccessive stages:

-   -   preparing at least one dispersion of silica in water,    -   bringing into contact and mixing an elastomer latex and the        aqueous silica dispersion in the presence of a calcium salt, in        order to obtain a coagulum,    -   recovering the coagulum,    -   drying the recovered coagulum in order to obtain the        masterbatch;    -   the molar content of calcium cations, defined as the number of        moles of calcium cations of the calcium salt per unit of silica        surface area (expressed in mol/m²), is between 1.19×10⁻⁶ and        2.81×10⁻⁶ mol/m².

It should be noted in particular that, surprisingly, small amounts ofcalcium salts coming within the abovementioned ranges, in the absence ofsilica dispersion, do not make it possible to produce coagulation of thediene elastomer latex alone.

Advantageously, the diene elastomer latex is a natural rubber latex andin particular a concentrated natural rubber latex.

According to one characteristic, the silica is a precipitated silica.

According to another characteristic, the amount of silica during theoperation in which two dispersions are brought into contact is between20 phr and 150 phr, parts per hundred parts by weight of elastomer,preferably between 30 phr and 100 phr, preferentially between 30 phr and90 phr and more preferably still between 30 phr and 70 phr.

According to one embodiment of the invention, the calcium salt ispreferably chosen from calcium sulphates, calcium halides, calciumcarbonates, calcium phosphates, calcium hydroxides, calcium oxalates andcalcium nitrates and, more preferably still, the calcium salt is acalcium chloride.

Another embodiment of the invention is a masterbatch of diene elastomerand silica prepared according to the method which comprises thefollowing successive stages:

-   -   preparing at least one dispersion of silica in water,    -   bringing into contact and mixing an elastomer latex and the        aqueous silica dispersion in the presence of a calcium salt,    -   recovering the coagulum,    -   drying the recovered coagulum in order to obtain the        masterbatch;    -   the molar content of calcium cations, defined as the number of        moles of calcium cations of the calcium salt per unit of silica        surface area (expressed in mol/m²), is between 1.19×10⁻⁶ and        2.81×10⁻⁶ mol/m².

A further subject-matter of the invention is a rubber composition basedon at least one masterbatch as mentioned above, and also a finished orsemi-finished article, a tire tread or a tire or semi-finished productcomprising at least one such rubber composition.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Measurements and Tests

Calculation of the Molar Content of Metal Cations

The molar content of calcium cations is defined as the number of molesof calcium cations of the calcium salt per unit of silica surface area(expressed in mol/m²), the unit of silica surface area being defined asthe weight of silica under consideration multiplied by the BET specificsurface.

Thus, for a weight w1 of metal salt, having a molar mass M1 and a numberN of metal atoms per salt molecule, added to a weight w2 of silica(irrespective of the fact that the silica is or is not already dispersedin aqueous solution) having a specific surface measured by BET of S2 (inm²/g), the molar content of metal cations, expressed in mol/m², is equalto: [(w1/M1)*N]/(w2*S2).

The BET specific surface is determined according to theBrunauer-Emmett-Teller method described in The Journal of the AmericanChemical Society, Vol. 60, page 309, February 1938, and corresponding toStandard NF T 45007.

Measurement of the Filler Content by TGA

This procedure has the object of quantifying the categories ofconstituents of the rubber mixtures. Three temperature intervals aredistinguished, each of which corresponds to a category of constituents:

-   -   between 250 at 550° C., corresponding to organic materials:        elastomers, oils, vulcanisation agents, and the like,    -   between 550 and 750° C., corresponding to intermediate losses        (carbon-based materials),    -   above 750° C., corresponding to ash and inorganic products: ZnO,        possibly silica, and the like.

It applies equally well to raw mixtures and to cured mixtures.

-   -   a) Equipment        -   Assembly for thermogravimetric analysis on a Mettler Toledo            analyser: model TGA 851 or TGA DSC1        -   1/100 mg balance, make and model of the balance        -   70 μl (coverless) alumina crucibles, Mettler Toledo, ref.            00024123        -   Various items of laboratory equipment: tongs, scissors, and            the like.    -   b) Principle

The weight losses of a mixture sample subjected to a temperature riseare monitored. The temperature rise takes place in 2 stages:

1/Heating from 25 to 550° C. under an inert atmosphere (N₂) in order toevaporate the volatile materials and to pyrolyse the organic materials.The volatility of the products which result therefrom brings about aweight loss corresponding, in a 1^(st) step (before 300° C.), to thevolatile materials and then to the organic materials initially presentin the mixture.

2/Continuing the heating up to 750° C. under an oxidizing atmosphere(air or O₂) in order to bring about the combustion of the black (and/orcarbon-based materials). The volatility of the products which resulttherefrom brings about a weight loss corresponding to the initial amountof black (and/or carbon-based materials).

The products which remain after these treatments constitute ash. Theyare generally inorganic materials of ZnO or silica type, and the like.

-   -   c) Measurements

c)-1—Preparation of the Samples

The amount of product analysed has to be weighed out to within 0.01 mgand has to be between 20 and 30 mg.

It is subsequently placed in a 70 μl (coverless) alumina crucible.

c)-2—Definition of the “method” (temperature program)

-   -   The following segments are successively defined:        -   1^(st) segment: dynamic from 25° C. to 550° C. at 50°            C./min, under nitrogen (40 ml/min)        -   2nd segment: dynamic from 550° C. to 750° C. at 10° C./min,            under air (or O₂)(40 ml/min)    -   The “blank curve subtraction” field is activated.

Any measurement is automatically corrected by a blank curve. The latteris produced under the same conditions as the measurement, with an emptycrucible. It is stored in memory and used for all the followingmeasurements (no new blank test necessary before each measurement).

c)-3—Setting the measurement under way

It is ascertained beforehand, by consulting the control window of thefurnace, that the nitrogen and air flow rates are suitably regulated (40μl/min) If not, they are adjusted using the adjustments located on the“gas box”.

-   -   Blank Curve    -   The blank curve is produced by following the procedure described        in the TGA operating manual.    -   Measurement    -   The measurement is carried out by following the procedure        described in the TGA operating manual.

c)-4—Use of the Curve

By following the instructions of the TGA operating manual

-   -   the curve to be used is selected and opened    -   the 1^(st) stationary phase, corresponding to the volatile        materials, is delimited on this curve between 25° C. and        approximately 250° C. respectively    -   the weight loss corresponding to the content of volatile        materials (in %) is calculated    -   the 2^(nd) stationary phase, corresponding to organic materials,        is delimited on this curve between the temperature of the 1^(5t)        stationary phase, approximately 250° C., and 550° C.        respectively    -   the weight loss corresponding to the content of organic        materials (in %) is calculated    -   the 3^(rd) stationary phase, corresponding to the losses, is        delimited on this curve between 550° C. and 750° C. respectively    -   the weight loss corresponding to these losses (in %) is        calculated    -   the residue or ash content (in %) is calculated.

c)-5—Presence of Volatile Compounds

For some mixtures comprising volatile compounds which can evaporate atambient temperature, there is a risk of losses of material between thepreparation of the sample and the actual start of the measurement.

These losses are not taken into account by the apparatus.

It is possible, in order to take these losses into account and to havethe true composition of the mixture, to proceed in the following way:

Stages c)-1 to c)-3 described above are carried out with the 2 followingset points:

-   -   during the preparation of the sample: the weight of the empty        crucible (W0) and the weight of the sample W1 are noted    -   when setting the measurement under way: the “crucible weight”        field is filled in with W0 and the “sample weight” field is        filled in with W1.

For the use (stage c)-4), the TGA takes into account, in order todetermine the losses, the weight of the sample W2, which it calculatesat the actual start of the measurement from the weight of the crucible,which is essential for the calculation of the residue; W2 is calculatedby the TGA by taking into account the weight W3 (crucible+sample) at thetime T0−W0.

The calculation of the contents of the different constituents and of theresidue is carried out with respect to the sample weight W1 definedduring the preparation and not with respect to W2.

The content of volatile materials then calculated by the apparatus iserroneous since a portion of VM, volatile materials, (W1−W2) hasevaporated during the wait between the preparation and the actual startof the measurement.

The VM values thus have to be recalculated manually:

-   -   by weight VM mg ═(W1−W2) mg+1^(st) stationary phase losses mg    -   by content Ct VM %=VM mg/P1×100 or 100−1^(st) stationary phase        residue %.

c)-6—Content of Filler in Pcom

This content is expressed in pcom, percentage of organic matter, and isobtained by the calculation, when the TGA measurement is interpreted,with the following formula:Ct filler(pcom)=[(D)/(B+C)]*100in which B represents the percentage of organic matter (interval between250 and 550° C.), C represents the percentage of intermediate losses(between 550 and 750° C.) and D represents the percentage of residue(above 750° C.).

Measurement of the Coagulation Yield

The coagulation yield corresponds to the ratio of the recovered dryweight (from which the weight of residual volatile materials afterdrying, as is defined in the TGA measurement protocol in the precedingsections, has been taken away) to the weight targeted at the start,multiplied by one hundred.

The method for preparing a masterbatch of diene elastomer and silicaaccording to embodiment of the invention comprises the followingsuccessive stages:

-   -   preparing at least one dispersion of silica in water,    -   bringing into contact and mixing an elastomer latex and the        aqueous silica dispersion in the presence of a calcium salt, in        order to obtain a coagulum,    -   recovering the coagulum,    -   drying the recovered coagulum in order to obtain the        masterbatch;        the molar content of calcium cations, defined as the number of        moles of calcium cations of the calcium salt per unit of silica        surface area (expressed in mol/m²), is between 1.19×10⁻⁶ and        2.81×10⁻⁶ mol/m².

Preparation of the Aqueous Silica Dispersion

Any silica (SiO₂) known to a person skilled in the art can be used forthe embodiments disclosed herein, in particular any precipitated orfumed silica exhibiting a BET specific surface and also a CTAB specificsurface both less than 450 m²/g, preferably from 30 to 400 m²/g.

It is possible in particular to use highly specific silicas (referred toas “HDSs”); mention will be made, for example, of the Ultrasil 7000 andUltrasil 7005 silicas from Degussa, the Zeosil 1165 MP, 1135 MP and 1115MP silicas from Rhodia, the Hi-Sil EZ150G silica from PPG, the Zeopol8715, 8745 and 8755 silicas from Huber or silicas having a high specificsurface, as described in Application WO 03/16837.

The silica is subsequently dispersed in water, preferably so as toobtain a dispersion from which the viscosity is sufficient to be easily“handleable”. Preferably, the concentration by weight of silica in thedispersion is between 0.1% and 30%. It will be possible, for example, toproduce an aqueous silica dispersion with a content of silica in thewater of 4% by weight.

Advantageously, the dispersion is sheared by sonication in order to makeit possible to stabilise the aggregates in the water, which makes itpossible to improve the dispersion of the silica in the masterbatchsubsequently produced.

This sonication can be carried out in particular using a 1500 wattVibracell generator manufactured by Sonics and Materials Inc., with aPZT (reference 75010) crystal piezoelectric converter, a booster for theprobe and a titanium alloy probe with a diameter of 19 mm (for a heightof 127 mm).

According to one embodiment of the invention, the calcium salt issubsequently added and the combined product is stirred using a spatula.It should be noted that it is advantageously possible to add the calciumsalt to the aqueous silica dispersion before the sonication stage, inorder in particular to improve the solubilization of the salt in theaqueous dispersion produced.

Calcium Salt

The calcium salts chosen from calcium sulphates, calcium halides, inparticular chlorides, bromides, fluorides and iodides, calciumcarbonates, calcium phosphates, calcium hydroxides, calcium oxalates andcalcium nitrates are particularly suitable and more preferably still thechosen calcium salt is a calcium chloride.

It is clear that it is possible to envisage using a blend of thedifferent abovementioned calcium salts.

Diene Elastomer Latex

As said above, the elastomer latex is a particular form of the elastomerwhich exists in the form of elastomer particles dispersed in water.

The disclosure relates to diene elastomer latexes, the diene elastomersbeing defined as follows:

“Diene” elastomer or rubber should be understood as meaning, in a knownway, an elastomer resulting at least in part (i.e., a homopolymer or acopolymer) from diene monomers (monomers carrying two carbon-carbondouble bonds which may or may not be conjugated).

These diene elastomers can be classified into two categories:“essentially unsaturated” or “essentially saturated”. “Essentiallyunsaturated” is understood to mean generally a diene elastomer resultingat least in part from conjugated diene monomers having a content ofunits of diene origin (conjugated dienes) which is greater than 15% (mol%); thus it is that diene elastomers such as butyl rubbers or copolymersof dienes and of α-olefins of EPDM type do not come within the precedingdefinition and can in particular be described as “essentially saturated”diene elastomers (low or very low content of units of diene origin,always less than 15%). In the category of “essentially unsaturated”diene elastomers, “highly unsaturated” diene elastomer is understood tomean in particular a diene elastomer having a content of units of dieneorigin (conjugated dienes) which is greater than 50%.

Furthermore, among these diene elastomers, a distinction is made betweennatural rubber and synthetic elastomers.

Natural rubber (NR), which is particularly suitable for embodiments ofthe invention, exists in different forms, as described in detail inChapter 3, “Latex concentrates: properties and composition”, by K. F.Gaseley, A. D. T. Gordon and T. D. Pendle, in “Natural Rubber Scienceand Technology”, A. D. Roberts, Oxford University Press, 1988.

In particular, several forms of natural rubber latex are commerciallyavailable: natural rubber latexes, referred to as field latexes,concentrated natural rubber latexes, epoxidized latexes (ENR),deproteinized latexes or prevulcanized latexes. Field natural rubberlatex is a latex to which ammonia has been added in order to preventpremature coagulation and concentrated natural rubber latex correspondsto a field latex which has been subjected to a treatment correspondingto washing, followed by concentrating. The various categories ofconcentrated natural rubber latex are listed in particular in StandardASTM D 1076-06. Among these concentrated natural rubber latexes, adistinction is made in particular between concentrated natural rubberlatexes of HA (high ammonia) grade and those of LA grade; use mayadvantageously be made, for embodiments of the invention, ofconcentrated natural rubber latexes of HA grade.

The latex can be used directly or can be diluted beforehand in water inorder to facilitate the use thereof.

Synthetic diene elastomers capable of being used in accordance withembodiments of the invention diene elastomer is understood to mean moreparticularly:

-   -   (a) —any homopolymer obtained by polymerization of a conjugated        diene monomer having from 4 to 12 carbon atoms;    -   (b) —any copolymer obtained by copolymerization of one or more        conjugated dienes with one another or with one or more        vinylaromatic compounds having from 8 to 20 carbon atoms;    -   (c) —a ternary copolymer obtained by copolymerization of        ethylene and of an α-olefin having from 3 to 6 carbon atoms with        a non-conjugated diene monomer having from 6 to 12 carbon atoms,        such as, for example, the elastomers obtained from ethylene and        propylene with a non-conjugated diene monomer of the        abovementioned type, such as, in particular, 1,4-hexadiene,        ethylidenenorbornene or dicyclopentadiene;    -   (d) —a copolymer of isobutene and of isoprene (butyl rubber) and        also the halogenated versions, in particular chlorinated or        brominated versions, of this type of copolymer.

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene or 2,4-hexadiene. The following, for example, aresuitable as vinylaromatic compounds: styrene, ortho-, meta- orpara-methyl styrene, ortho, the “vinyltoluene” commercial mixture,para-(tert-butyl)styrene, methoxystyrenes, chlorostyrenes,vinylmesitylene, divinylbenzene or vinyl-naphthalene.

The copolymers can comprise between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinylaromatic units. The elastomerscan have any microstructure which depends on the polymerizationconditions used, in particular on the presence or absence of a modifyingand/or randomizing agent and on the amounts of modifying and/orrandomizing agent employed. The elastomers can, for example, be block,random, sequential or microsequential elastomers and can be prepared indispersion or in solution; they can be coupled and/or star-branched oralso functionalized with a coupling and/or star-branching orfunctionalization agent. For coupling with carbon black, mention may bemade, for example, of functional groups comprising a C—Sn bond or ofaminated functional groups, such as aminobenzophenone, for example; forcoupling with a reinforcing inorganic filler, such as silica, mentionmay be made, for example, of silanol or polysiloxane functional groupshaving a silanol end (such as described, for example, in FR 2 740 778 orU.S. Pat. No. 6,013,718, and WO 2008/141702), of alkoxysilane groups(such as described, for example, in FR 2 765 882 or U.S. Pat. No.5,977,238), of carboxyl groups (such as described, for example, in WO01/92402 or U.S. Pat. No. 6,815,473, WO 2004/096865 or US 2006/0089445)or of polyether groups (such as described, for example, in EP 1 127 909or U.S. Pat. No. 6,503,973, WO 2009/000750 and WO 2009/000752). Mentionmay also be made, as other examples of functionalized elastomers, ofelastomers (such as SBR, BR, NR or IR) of the epoxidized type.

The following are suitable: polybutadienes, in particular those having acontent (mol %) of 1,2-units of between 4% and 80% or those having acontent (mol %) of cis-1,4-units of greater than 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a Tg (glasstransition temperature (Tg, measured according to ASTM D3418) of between0° C. and −70° C. and more particularly between −10° C. and −60° C., astyrene content of between 5% and 60% by weight and more particularlybetween 20% and 50%, a content (mol %) of 1,2-bonds of the butadienepart of between 4% and 75% and a content (mol %) of trans-1,4-bonds ofbetween 10% and 80%, butadiene/isoprene copolymers, in particular thosehaving an isoprene content of between 5% and 90% by weight and a Tg of−40° C. to −80° C., or isoprene/styrene copolymers, in particular thosehaving a styrene content of between 5% and 50% by weight and a Tg ofbetween −5° C. and −50° C. In the case of butadiene/styrene/isoprenecopolymers, those having a styrene content of between 5% and 50% byweight and more particularly of between 10% and 40%, an isoprene contentof between 15% and 60% by weight and more particularly between 20% and50%, a butadiene content of between 5% and 50% by weight and moreparticularly of between 20% and 40%, a content (mol %) of 1,2-units ofthe butadiene part of between 4% and 85%, a content (mol %) oftrans-1,4-units of the butadiene part of between 6% and 80%, a content(mol %) of 1,2-plus 3,4-units of the isoprene part of between 5% and 70%and a content (mol %) of trans-1,4-units of the isoprene part of between10% and 50%, and more generally any butadiene/styrene/isoprene copolymerhaving a Tg of between −5° C. and −70° C., are suitable in particular.

To sum up, the synthetic diene elastomer or elastomers according toembodiments of the invention are preferably selected from the group ofthe highly unsaturated diene elastomers consisting of polybutadienes(abbreviated to “BRs”), synthetic polyisoprenes (IRs), butadienecopolymers, isoprene copolymers and the mixtures of these elastomers.Such copolymers are more preferably selected from the group consistingof butadiene/styrene copolymers (SBRs), isoprene/butadiene copolymers(BIRs), isoprene/styrene copolymers (SIRs) andisoprene/butadiene/styrene copolymers (SBIRs).

Thus, as synthetic elastomer latex, the latex can in particular consistof a synthetic diene elastomer already available in the emulsion form(for example, a copolymer of butadiene and of styrene, SBR, prepared inemulsion) or of a synthetic diene elastomer initially in solution (forexample an SBR prepared in solution) which is emulsified in a mixture oforganic solvent and water, generally using a surface-active agent.

A latex of SBR, in particular an SBR prepared in emulsion (“ESBR”) or anSBR prepared in solution (“SSBR”), more particularly an SBR prepared inemulsion, is particularly suitable for the embodiments disclosed herein.

There exist two main types of processes for the emulsioncopolymerization of styrene and of butadiene, one of them, or hotprocess (carried out at a temperature of approximately 50° C.), beingsuitable for the preparation of highly branched SBRs while the other, orcold process (carried out at a temperature which can range from 15° C.to 40° C.), makes it possible to obtain more linear SBRs.

For a detailed description of the effectiveness of several emulsifierswhich can be used in the said hot process (as a function of the contentsof the said emulsifiers), reference may be made, for example, to the twopapers by C. W. Carr, I. M. Kolthoff and E. J. Meehan, University ofMinnesota, Minneapolis, Minn., which appeared in the Journal of PolymerScience in 1950, Vol. V, No. 2, pp. 201-206, and in 1951, Vol. VI, No.1, pp. 73-81.

As regards comparative examples of implementation of the said coldprocess, reference may be made, for example, to the paper Industrial andEngineering Chemistry, 1948, Vol. 40, No. 5, pp. 932-937, E. J.Vandenberg and G. E. Hulse, Hercules Powder Company, Wilmington, Del.,and to the paper Industrial and Engineering Chemistry, 1954, Vol. 46,No. 5, pp. 1065-1073, J. R. Miller and H. E. Diem, B. F. GoodrichChemical Co., Akron, Ohio.

In the case of an SBR (ESBR or SSBR) elastomer, use is made inparticular of an SBR having a moderate styrene content, for example ofbetween 20% and 35% by weight, or a high styrene content, for examplefrom 35 to 45%, a content of vinyl bonds of the butadiene part ofbetween 15% and 70%, a content (mol %) of trans-1,4-bonds of between 15%and 75% and a Tg of between −10° C. and −55° C.; such an SBR canadvantageously be used as a mixture with a BR preferably having morethan 90% (mol %) of cis-1,4-bonds.

It should be noted that it is possible to envisage using one or morenatural rubber latexes as a blend, one or more synthetic rubber latexesas a blend or a blend of one or more natural rubber latexes with one ormore synthetic rubber latexes.

Bringing the Two Dispersions into Contact

The two dispersions are brought into contact in the presence of acalcium salt.

Advantageously, the salt is added to the aqueous silica dispersion aswas explained above, that is to say during the formation of the aqueoussilica dispersion or after its formation. However, it would also bepossible to envisage adding the calcium salt during the operation inwhich the aqueous silica dispersion and the elastomer latex are broughtinto contact. In the latter case, the salt can advantageously beintroduced in the form of a saline solution.

In order to make possible the satisfactory mixing of these solutions,the latter are, for example, poured into a beaker with magneticstirring.

It is also possible to use any type of device which makes possible“efficient” mixing of two products in the liquid phase; thus, it will bepossible to use a static mixer, such as static mixers sold by NoritakeCo. Limited, TAH in the USA, Koflo in the USA, or Tokushu Kika Kogyo Co.Ltd, or a mixture which produces high shear, such as mixers sold byTokushu Kika Kogyo Co. Ltd, by PUC in Germany, by Cavitron in Germany orby Silverson in the United Kingdom.

It is clear that, the more efficient the mixing stage, the better thedispersion and the homogeneity of the silica-diene elastomermasterbatch.

During this phase of mixing the two dispersions, a coagulum of elastomerand of silica is formed, either in the form of just one solid componentin the solution or in the form of several separate solid components.

Surprisingly, it has been found that, in order to actually obtain acoagulum, with a coagulation yield of greater than or equal to 80% andwhich corresponds to the production of a masterbatch observing theinitial ratio by weight of filler with respect to the elastomer, adifference of 20% with respect to the ratio initially calculated beingregarded as acceptable, the molar content of calcium cations, defined asthe number of moles of calcium cations of the calcium salt per unit ofsilica surface area (expressed in mol/m²), has to be between 1.19×10⁻⁶and 2.81×10⁻⁶ mol/m².

The volumes of the two dispersions to be brought into contact and inparticular the volume of silica dispersion depend on the silica contenttargeted for the masterbatch to be produced, taking into account, ofcourse, the respective concentrations of the dispersions. Thus, thevolume will be accordingly adapted. Advantageously, the silica contenttargeted for the masterbatch is between 20 and 150 phr (parts by weightper hundred parts of elastomer), preferably between 30 and 100 phr andmore preferably between 30 and 90 phr, more preferably still between 30and 70 phr.

Recovery of the Solid Formed

The solid or solids recovered are filtered or centrifuged. This isbecause the filtering operation, which can be carried out using afiltration sieve or cloth, may prove to be unsuitable when the coagulumexists in the form of numerous small solid components. In such a case,an additional centrifuging operation is preferably carried out.

On conclusion of this filtering or centrifuging stage, the coagulumobtained is dried, for example in an oven.

On conclusion of the drying stage, the dry coagulum obtained canadvantageously be homogenized in order to ensure that the samplewithdrawn for the measurement of the content of filler isrepresentative, for example by carrying out gentle mechanical working onan open mill.

On conclusion of this operation, the content of filler is measured byTGA and the coagulation yield is also measured.

Rubber Composition

Advantageously, the masterbatches thus produced are capable of beingused in rubber compositions, in particular for tires.

The rubber compositions for tires based on the masterbatches accordingto embodiments of the invention can also comprise, in a known way, acoupling agent and a vulcanization system.

It should be remembered here that the term “coupling agent” isunderstood to mean, in a known way, an agent capable of establishing asatisfactory connection, of chemical and/or physical nature, between theinorganic filler and the diene elastomer; such a coupling agent, whichis at least bifunctional, has, for example, the simplified generalformula “Y—Z—X”, in which:

-   -   Y represents a functional group (“Y” functional group) which is        capable of being bonded physically and/or chemically to the        inorganic filler, it being possible for such a bond to be        established, for example, between a silicon atom of the coupling        agent and the surface hydroxyl (—OH) groups of the inorganic        filler (for example the surface silanols, when silica is        concerned);    -   X represents a functional group (“X” functional group) capable        of being bonded physically and/or chemically to the diene        elastomer, for example via a sulphur atom;    -   Z represents a divalent group which makes it possible to connect        Y and X.

Coupling agents, in particular silica/diene elastomer coupling agents,have been described in a very large number of documents, the most wellknown being bifunctional organosilanes bearing alkoxyl functional groups(that is to say, by definition, “alkoxysilanes”) as “Y” functionalgroups and, as “X” functional groups, functional groups capable ofreacting with the diene elastomer, such as, for example, polysulphidefunctional groups.

Mention should in particular be made, among the known alkoxysilanepolysulphide compounds, of bis(3-triethoxysilylpropyl)tetrasulphide(abbreviated to “TESPT”), of formula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, sold inparticular by Degussa under the name “Si69” (or “X50S”, when it issupported at 50% by weight on carbon black), in the form of a commercialmixture of polysulphides S_(x) with a mean value for x which isapproximately 4.

It should be noted that it is possible to envisage the introduction of acoupling agent in the preparation of the masterbatch in order todirectly obtain a masterbatch of elastomer and silica also comprising acoupling agent. The coupling agent can thus be added before or duringthe operation in which the aqueous silica dispersion and the dieneelastomer latex are brought into contact.

These rubber compositions in accordance with embodiments of theinvention can also comprise all or a portion of the usual additivesgenerally used in the elastomer compositions intended for themanufacture of tires, in particular of treads, such as, for example,plasticizers or extending oils, whether the latter are aromatic ornonaromatic in nature, pigments, protection agents, such as antiozonewaxes, chemical antiozonants or antioxidants, antifatigue agents,reinforcing resins, methylene acceptors (for example phenolic novolakresin) or methylene donors (for example HMT or H3M), as described, forexample, in Application WO 02/10269, a crosslinking system based eitheron sulphur or on sulphur donors and/or on peroxide and/or onbismaleimides, or vulcanization accelerators.

Preferably, these compositions comprise, as preferred nonaromatic orvery slightly aromatic plasticizing agent, at least one compoundselected from the group consisting of naphthenic oils, paraffinic oils,MES oils, TDAE oils, glycerol esters (in particular trioleates),plasticizing hydrocarbon resins exhibiting a high Tg preferably ofgreater than 30° C., and the mixtures of such compounds.

These compositions can also comprise, in addition to the couplingagents, coupling activators, covering agents (for example comprisingonly the Y functional group) for the reinforcing inorganic filler ormore generally processing aids capable, in a known way, by virtue of animprovement in the dispersion of the inorganic filler in the rubbermatrix and of a lowering of the viscosity of the compositions, ofimproving their property of processing in the raw state, these agentsbeing, for example, hydrolysable silanes, such as alkylalkoxysilanes (inparticular alkyltriethoxysilanes), polyols, polyethers (for examplepolyethylene glycols), primary, secondary or tertiary amines (forexample trialkanolamines), hydroxylated or hydrolysable POSs, forexample α,Ω-dihydroxy-polyorganosiloxanes (in particularα,Ω-dihydroxypolydimethylsiloxanes), or fatty acids, such as, forexample, stearic acid.

It could also be possible to envisage incorporating, in the masterbatch,before the formation of the coagulum, the additives described above,oil, antioxidant, covering agent, and the like.

Manufacture of the Rubber Compositions

The rubber compositions of embodiments of the invention are manufacturedin appropriate mixers, using two successive phases of preparationaccording to a general procedure well known to a person skilled in theart: a first phase of thermomechanical working or kneading (sometimesreferred to as “non-productive” phase) at high temperature, up to amaximum temperature of between 130° C. and 200° C., preferably between145° C. and 185° C., followed by a second phase of mechanical working(sometimes referred to as “productive” phase) at lower temperature,typically below 120° C., for example between 60° C. and 100° C., duringwhich finishing phase the crosslinking or vulcanization system isincorporated.

According to a preferred embodiment of the invention, all the baseconstituents of the compositions, with the exception of thevulcanization system, namely the masterbatch, the coupling agent (if itis not already present in the masterbatch) and the carbon black, ifappropriate, are intimately incorporated, by kneading, in the dieneelastomer during the first “non-productive” phase, that is to say thatat least these various base constituents are introduced into the mixerand that they are thermomechanically kneaded, in one or more stages,until the maximum temperature of between 130° C. and 200° C., preferablyof between 145° C. and 185° C., is reached.

By way of example, the first (non-productive) phase is carried out in asingle thermomechanical stage during which all the necessaryconstituents, the optional additional covering agents or processing aidsand various other additives, with the exception of the vulcanizationsystem, are introduced into an appropriate mixer, such as an ordinaryinternal mixer. The total duration of the kneading, in thisnon-productive phase, is preferably 1 and 15 min. After cooling themixture thus obtained during the first non-productive phase, thevulcanization system is then incorporated at low temperature, generallyin an external mixer, such as an open mill; everything is then mixed(productive phase) for a few minutes, for example between 2 and 15 min.

When a covering agent is used, it can be incorporated completely duringa non-productive phase, at the same time as the inorganic filler, orelse completely during the productive phase, at the same time as thevulcanization system, or also split up over the two successive phases.

It should be noted that it is possible to introduce all or a portion ofthe covering agent in a form supported (it being placed on the supportbeforehand) or a solid compatible with the chemical structurescorresponding to this compound. For example, when splitting up betweenthe two successive phases above, it can be advantageous to introduce thesecond portion of the covering agent, on the external mixer, after beingplaced on a support in order to facilitate the incorporation thereof andthe dispersion thereof.

The crosslinking system is preferably a vulcanization system, that is tosay a system based on sulphur (or on a sulphur-donating agent) and on aprimary vulcanization accelerator. Additional to this base vulcanizationsystem are, incorporated during the first non-productive phase and/orduring the productive phase, as are described subsequently, variousknown secondary vulcanization accelerators or vulcanization activators,such as zinc oxide, stearic acid or equivalent compounds, or guanidinederivatives (in particular diphenylguanidine).

The sulphur is used at a preferred content of between 0.5 and 12 phr, inparticular between 1 and 10 phr. The primary vulcanization acceleratoris used at a preferred content of between 0.5 and 10 phr, morepreferably of between 0.5 and 5.0 phr.

Use may be made, as (primary or secondary) accelerator, of any compoundcapable of acting as accelerator of the vulcanization of dieneelastomers in the presence of sulphur, in particular accelerators of thethiazole type and their derivatives, accelerators of thiuram type oraccelerators of zinc dithiocarbamate type. These accelerators are, forexample, selected from the group consisting of 2-mercaptobenzothiazyldisulphide (abbreviated to “MBTS”), tetrabenzylthiuram disulphide(“TBZTD”), N-cyclohexyl-2-benzothiazolesulphenamide (“CBS”),N,N-dicyclohexyl-2-benzothiazolesulphenamide (“DCBS”),N-(tert-butyl)-2-benzothiazolesulphenamide (“TBBS”),N-(tert-butyl)-2-benzothiazolesulphenimide (“TBSI”), zincdibenzyldithiocarbamate (“ZBEC”) and the mixtures of these compounds.

The final composition thus obtained is subsequently calendered, forexample in the form of a sheet or a plaque, in particular for laboratorycharacterization, or also extruded in the form of a rubber profiledelement which can be used, for example, as a passenger vehicle tiretread.

EXAMPLES Preparation of the Masterbatches

Equipment

-   -   preparation of the aqueous silica dispersion    -   Thermobalance of HR73 Halogen Moisture Analyser type from        Mettler Toledo (confirmation of volatile material content of the        silica)    -   Balance of PG5002-S Delta Range® type (max. 5100 g, accuracy of        0.1/0.01 g) from Mettler Toledo (weighings silica+water)    -   100 ml low-form glass beaker    -   Balance of AG204 type (max. 210 g, accuracy of 0.1 mg) from        Mettler Toledo (for weighing the salt)    -   1500 Watt Vibracell ultrasound generator manufactured by Sonics        and Materials Inc. (ref.: VC1500, 20 kHz), with a PZT crystal        piezoelectric converter (reference CV154), a booster for the        probe (ref. BHN15GD, amplitude increased by 50%) and a standard        non-threaded probe (not high gain) made of titanium alloy with a        diameter of 19 mm (for a height of 127 mm)    -   Magnetic stirrer of MR3003 type from Heidolph    -   Straight magnetic bar with a length of 25 mm and a diameter of 8        mm    -   preparation of the latex    -   Thermobalance of HR73 Halogen Moisture Analyser type from        Mettler Toledo (confirmation of volatile material content of the        latex)    -   Balance of PG5002-S Delta Range® type (max. 5100 g, accuracy of        0.01 g) from Mettler Toledo (weighings latex+water)    -   250 ml low-form glass beaker    -   bringing into contact    -   Magnetic stirrer of MR3003 type from Heidolph    -   Straight magnetic bar with a length of 25 mm and a diameter of 8        mm    -   recovery    -   Centrifuge of 4K15 type from Sigma Laboratory Centrifuges        Bioblock Scientific, equipped with a Sigma rotor, reference No.        12256 (max. 6×400 g)    -   250 ml Nalgene® bottle

Reactants

silica:

precipitated silica 160 MP (Zeosil 1165 MP), manufactured by Rhodia,with a BET specific surface measured at 160 m²/g, with a true density of2.14 g/cm³, with a content of “surface” silanol functional groups(measured by Me(OH) grafting) of 7.4 OH/nm² and a “total” content ofsilanol functional groups (measured by TGA) of 120H/nm²,

Salt:

calcium chloride (CaCl₂.2H₂O), purity greater than or equal to 99.0% byweight, ACS Reagent grade, CAS 110035-04-81, reference 223506-500G, soldby Aldrich,

Latex:

natural rubber latex of HA grade, sold by Trang Latex/Beetex (Thailand),having a solids content measured at 61.12%, pH of approximately 10.65,comprising approximately 20-25 ppm of Mg²⁺,

Water:

ultrapure water (18.2 MΩcm).

Procedure

-   -   Preparation of a Masterbatch of 10.5 g in Theory

For the examples, an amount of silica of 50 parts by weight per hundredparts of elastomer was chosen, which corresponds here to 50 pcom (thisis because the masterbatches described here comprise only silica and thediene elastomer).

-   -   preparation of the aqueous silica dispersion (4% by weight)

3.5 dry g equivalents of silica are weighed out and approximately 84 gof ultrapure water are added. Of course, the amount of absorbed waterpresent in the silica has to be taken into account; thus, for example,3.5 dry g equivalent of silica comprising 6.36% by weight of volatilematerial=3.74 g of silica to be weighed out and the 84 g of water inorder to obtain a 4% by weight slurry become 84-(3.74-3.5)=83.76 g.

The amount of calcium chloride is weighed out and is introduced into theaqueous silica suspension (brought into contact for at least 10minutes).

The [silica+water+salt] suspension is subjected to ultrasound for 8minutes at 60% of the maximum power of the ultrasound generator, the 100ml beaker being maintained at a set temperature by being immersed in anice bath.

The dispersion thus obtained, correctly subjected to ultrasound, issubsequently kept magnetically stirred (500 rev/min) until thecontacting operation.

-   -   preparation of the latex (30% by weight)

The latex is weighed out and its concentration is adjusted (dilution byaddition of ultrapure water).

-   -   bringing into contact

The dispersion obtained above [silica+water+salt] is poured into thelatex with magnetic stirring (500 rev/min)

The combined mixture is stirred at 500 rev/min for 3 minutes.

-   -   recovery

In order to have identical conditions of the procedure for the varioustests, the coagulum formed or the solids formed (currently referred toas “crumbs”) are centrifuged, including in the cases where the visualappearance of the coagulum allowed a filtering operation to beenvisaged.

Centrifuging is carried out, after transfer into a 250 ml Nalgenebottle, using a Sigma 4K15 bucket centrifuge at 8000 revolutions/min for10 minutes.

The coagulum (in the form of a pellet) thus recovered is dried in a fumecupboard at ambient temperature for 24 hours and then under vacuum,under 100 to 300 mbar, in an oven at 65° C. for 24 h, in order to removethe final traces of water.

The content of filler is then measured by TGA and the coagulation yieldis also measured on the masterbatch thus produced. Advantageously, agentle mechanical working can be carried out, such as on an open mill,in order to homogenize the coagulum and to ensure that the sampling forthe measurement is representative. It is also possible to carry outseveral TGA measurements on the coagulum (for example 10 measurements)in order to produce the mean thereof in order to have a valuerepresentative of the whole of the coagulum.

Example

The aim of this example is to demonstrate the satisfactory operation ofthe method in accordance with the invention, in particular with respectto the molar content of calcium cations, the calcium salt used in thisexample being a calcium chloride CaCl₂.2H₂O, as specified in sectionIII-1.

Tests E1, E2, E3 and E4 are carried out in accordance with the processdescribed in detail in the preceding section, with:

-   -   a high ammonia concentrated natural rubber latex originating        from the supplier Trang Latex/Beetex (Thailand), having a solids        content measured at 61.12%, as specified in section III-1, and        which was diluted twofold,    -   an amount of silica, during the operation in which the two        dispersions are brought into contact, of 50 phr.

The only difference between these four tests consists, during theprocedure described in detail above, of the amount of calcium cationsintroduced during the addition of calcium salt to the aqueous silicadispersion.

Thus, tests E1, E2 and E3 differ from one another as follows:

-   -   for E1, the molar content of calcium cations is 1.12×10⁻⁶        (corresponding to a weight of calcium chloride hydrate of 0.097        g for a targeted masterbatch of 10.5 g, comprising 50 parts by        weight per hundred parts of elastomer targeted),    -   for E2, the molar content of calcium cations is 1.56×10⁻⁶        (corresponding to a weight of calcium chloride hydrate of 0.130        g),    -   for E3, the molar content of calcium cations is 2.31×10⁻⁶        (corresponding to a weight of calcium chloride hydrate of 0.195        g),    -   for E4, the molar content of calcium cations is 3.12×10⁻⁶        (corresponding to a weight of calcium chloride hydrate of 0.259        g).

The results obtained (yield and content of filler) for these four testsare presented in the following Table 1:

TABLE 1 Yield Content of filler Test (% by weight) (pcom) E1 — — E2 95.650.9 E3 95.4 47.7 E4 74.3 26.5

It is found that test E1, not in accordance with the invention, did notmake possible the coagulation of the elastomer with the silica. A phaseseparation of the silica and the latex occurred during the stage ofrecovery by centrifuging; no coagulum was thus obtained.

For tests E2 and E3 in accordance with the invention, both acceptablecontents of silica (between 40 pcom and 60 pcom) and a yield of greaterthan 80% were obtained.

For test E4 not in accordance with the invention, it is found that, eventhough a masterbatch was obtained, the yield is at the limit ofacceptability and in particular the content of silica (26.5 pcom) isoutside the acceptable contents.

It is clearly apparent that it is important to be within a given rangeof molar content of calcium cations in order to meet the criteriadesired in terms of content of filler observed and yield obtained.

The invention claimed is:
 1. A method for preparing a masterbatch ofdiene elastomer and silica comprising: preparing at least one dispersionof silica in water, bringing into contact and mixing an elastomer latexand the aqueous silica dispersion in the presence of an at leastdivalent calcium salt, thereby obtaining a coagulum, recovering thecoagulum, drying the recovered coagulum, thereby obtaining amasterbatch; wherein the molar content of calcium cations, defined asthe number of moles of calcium cations of the calcium salt per unit ofsilica surface area, is between 1.19×10⁻⁶ and 2.81×10⁻⁶ mol/m².
 2. Themethod according to claim 1, wherein the at least divalent calcium saltis added to the aqueous silica dispersion before the latter is broughtinto contact with the elastomer latex.
 3. The method according to claim1, wherein recovering of the coagulum comprises filtering.
 4. The methodaccording to claim 1, wherein the recovering of the coagulum comprisescentrifuging.
 5. The method according to claim 1, wherein the dieneelastomer latex is a natural rubber latex.
 6. The method according toclaim 5, wherein the natural rubber latex is a concentrated naturalrubber latex.
 7. The method according to claim 6, wherein the silica isa precipitated silica.
 8. The method according to claim 1, wherein thesilica present during the bringing of the two dispersions into contactis an amount between 20 phr and 150 phr, parts per hundred parts byweight of elastomer.
 9. The method according to claim 8, wherein thesilica present during the bringing of the two dispersions into contactis an amount between 30 phr and 100 phr.
 10. The method according toclaim 9, wherein the amount of silica present during the bringing of thetwo dispersions into contact is an amount between 30 phr and 90 phr. 11.The method according to claim 10, wherein the calcium salt is selectedfrom the group consisting of calcium sulphates, calcium halides andcalcium nitrates.
 12. The method according to claim 11 wherein thecalcium salt is an calcium chloride.
 13. The method according to claim12, further comprising adding an aqueous dispersion of coupling agentbefore or during bringing the aqueous silica dispersion and the dieneelastomer latex into contact.
 14. A masterbatch of diene elastomer andsilica prepared according to the method of claim
 1. 15. A rubbercomposition based on at least one masterbatch according to claim
 14. 16.A finished or semi-finished article comprising the rubber compositionaccording to claim
 15. 17. A tire tread comprising the rubbercomposition according to claim
 15. 18. A tire or semi-finished productcomprising at least one rubber composition according to claim 15.